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Ethernet ports

File: src/topics/ethernet.j. Path: /interface/ethernet (ETHERNET_PATH).

Background

This is the physical layer: the copper (or SFP) ports themselves. Everything else - bridges, VLANs, addresses - rides on top of a link that has to come up first, at the right speed and duplex. Two facts do most of the explaining here:

  1. Speed and duplex are negotiated. Both ends advertise what they can and agree on the best match. That works so well that the only reason to interfere is a link partner that cannot negotiate - and if you force one side, you must force the other identically, or you get a duplex mismatch: the link comes up fine and then crawls, with errors under load. It is the classic "network is slow" root cause.
  2. A port serving a bridge is a slave. Most of its own settings stop mattering; configure the bridge instead.

On PoE-capable models the port also powers the device at the far end - which makes the PoE mode a remote power switch.

Structs

jennifer
mt.EthernetPort {
    id, name, defaultName,   # defaultName is the stable factory name
    mac, mtu,
    autoNegotiation,         # bool
    poeOut,                  # "auto-on" / "forced-on" / "off", "" = no PoE
    running, slave, disabled, comment
}
mt.LinkStatus {
    name, up, status,        # up = status == "link-ok"
    rate, fullDuplex,        # what was actually negotiated
    autoNegotiation          # "done" / "incomplete" / "disabled"
}

Functions

FunctionPurpose
ethernetPorts(c) → list of EthernetPortall physical ports
ethernetPortByName(c, name) → EthernetPortone port
linkStatus(c, name) → LinkStatusmeasure the link right now
forceEthernetLink(c, name, speed, fullDuplex)pin speed/duplex (both ends!)
autoNegotiateEthernet(c, name)back to the healthy default
setEthernetMtu(c, name, mtu)change the MTU (68-65535)
setPoe(c, name, mode)PoE: auto-on / forced-on / off

Enable/disable, rename, and comment work through the interface topic (disableInterface etc.) - ethernet ports are interfaces too.

Examples

The "why is it slow" check across all ports:

jennifer
def ports as list of mt.EthernetPort init mt.ethernetPorts($c);
for (def p in $ports) {
    if ($p.running) {
        def ls as mt.LinkStatus init mt.linkStatus($c, $p.name);
        io.printf("%s: %s %s duplex=%t\n", $p.name, $ls.status, $ls.rate, $ls.fullDuplex);
    }
}

A stubborn industrial device that cannot negotiate:

jennifer
mt.forceEthernetLink($c, "ether4", "100Mbps", true);
# the device end must be set to 100/full as well!
# ... and back to normal later:
mt.autoNegotiateEthernet($c, "ether4");

Power-cycle a hung PoE access point from your desk:

jennifer
mt.setPoe($c, "ether5", "off");
# give it a few seconds (e.g. via your own loop or scheduler)
mt.setPoe($c, "ether5", "auto-on");

Jumbo frames towards a storage network:

jennifer
mt.setEthernetMtu($c, "ether3", 9000);
# every device on that path must agree on 9000, and the port's
# hardware limit (l2mtu) caps it - the router refuses what the
# chip cannot do.

Pitfalls

  • Never force just one end. Duplex mismatch does not break the link; it breaks the throughput, which is far harder to spot. When in doubt: autoNegotiateEthernet on both sides.
  • linkStatus measures; ethernetPorts reads configuration. A port configured for auto-negotiation may still be down - check up.
  • A slave == true port belongs to a bridge; its speed/duplex still matter (physical layer), but addresses and most protocols live on the bridge.
  • setPoe(..., "forced-on") powers the port even with nothing detected - fine for odd devices that fail detection, risky for everything else. Prefer auto-on.
  • Renamed ports keep their defaultName ("ether1") - use it to find the physical port a creative admin renamed.